The invention relates to a high pressure discharge lamp comprising:
a light-transmitting discharge vessel, which is sealed in a gas-tight manner, which has an ionizable gas filling and in which discharge electrodes are present, connected to current supplies entering the discharge vessel; PA1 a light-transmitting outer envelope, which is sealed in a gas-tight manner and which surrounds the discharge vessel; PA1 current conductors entering the outer envelope and being connected to a respective current supply; PA1 an oxygen dispenser containing an oxygen compound disposed in the outer envelope, to release oxygen into the outer envelope upon the oxygen compound being decomposed by heat.
Such a high pressure discharge lamp is known from U.S. Pat. No. 4,918,352.
The known lamp has in the outer envelope either an oxygen containing gas filling or an oxygen dispenser, which releases oxygen by heat evolved upon the lamp being switched on. According to said patent specification said measure is taken to oxidize the surface of the current conductors and thereby to prevent the loss of sodium from the gas filling of the discharge vessel.
It is a disadvantage of gaseous oxygen being present inside the outer envelope immediately after completing the lamp manufacture, that the gas-tightness of the outer envelope can not be verified by generating a glow discharge inside the outer envelope. So, an oxygen dispenser would be advantageous, which releases oxygen only upon being heated after the gas tightness of the outer envelope has been verified. The said patent specification does, however, unfortunately not mention any oxygen compound pound which could be used for said purpose.
From U.S. Pat. No. 4,499,396, it is known to be advantageous to have a slightly oxidizing gas filling in the outer envelope due to the presence of a trace of oxygen, in order to prevent a phosphor coating on the inside surface of the outer envelope to become reduced and as a result to become blackened. Blackening causes a decrease of the luminous maintenance of the lamp. The presence of oxygen gas in the outer envelope immediately after the completion of the lamp manufacture to prevent this, is a disadvantage, however.
Nevertheless, there is a strong felt need to obviate blackening of the outer envelope of high pressure discharge lamps. Such blackening may occur as a result of hydrocarbons being present in the outer envelope. Already during the first hours of operation of lamps hydrocarbons are decomposed to give carbon which deposits as a black layer on the outer envelope and/or the discharge vessel. A black layer does not influence the luminous maintenance, only, but also the temperature of the discharge vessel, which may result in a color shift. As these deposits occur within a few hours of operation already, they have a long lasting negative effect on the properties of lamps. So, it is highly desirable to combat the occurrence of carbon deposits as fast as is possible.
Hydrocarbons in lamps may originate from several sources. They may have been introduced into the outer envelope as contaminations on lamp parts, e.g. on its current conductors, or originate from the oil in the vacuum pump used to evacuate the outer envelope, eventually prior to it being filled with inert gas, such as e.g. Ne/N.sub.2. They also may be a residue of a binder, e.g. a binder used to bring about a coating, such as a heat conserving coating, such as a coating of zirconium oxide, on end portions of the discharge vessel, or a binder to make a phosphor coating. Apart from causing a black deposit which hampers the transmission of light and reduces the luminous maintenance, and which possibly causes a color shift, carbon originating from hydrocarbons may reduce the phosphor coating, if present, causing the coating to blacken and making the coating less effective.
APL Engineered Materials, Inc, of Illinois U.S.A. discloses in its Product Development Information Bulletin: Metal Halide Lamp Getter, of Dec. 1, 1989, a metal halide lamp being provided in its outer envelope with a stainless steel case having a porous cover and containing a disc of barium peroxide disposed at a temperature of between 200 and 360.degree. C. The getter maintains a slightly oxidizing atmosphere within the outer envelope. This is said to be particularly advantageous for lamps that are sensitive to hydrocarbon contamination, such as lamps with a phosphor coated outer envelope.
BaO.sub.2 as an oxygen generator in an oxygen dispenser was found to be of little value. BaO.sub.2 releases oxygen that reacts with hydrocarbons according to the following reactions: EQU BaO.sub.2 .fwdarw.BaO+1/2O.sub.2 (I) EQU C.sub.n H.sub.m+(n+1/4 m)O.sub.2 .fwdarw.n CO.sub.2 +(m/2) H.sub.2 O (II)
The use of BaO.sub.2 has, however, some technical drawbacks.
First, BaO.sub.2 reacts with hydrogen, generally present in lamps, according to the reaction: EQU BaO.sub.2 +H.sub.2 .fwdarw.Ba(OH).sub.2 (III)
The use of BaO.sub.2 in lamps had been originally proposed by U.S. Pat. No. 3,519,864, with the very aim of absorbing hydrogen which has negative effects on the discharge voltage in the discharge vessel.
Thus formed Ba(OH).sub.2, in turn, may decompose according to the reaction: EQU Ba(OH).sub.2 BaO+H.sub.2 O (IV)
which reaction is undesirable.
Moreover, reactions (I), (III) and (IV) may take place simultaneously, thus making difficult an exact dosing of BaO.sub.2. Dosing is made even more complicated by the fact that the rate of these reactions have a different temperature dependency. To obviate this problem, APL commercial bulletin states that BaO.sub.2 container positioning inside the lamp must be such that BaO.sub.2 is kept at a temperature comprised between about 250 and 360.degree. C. BaO.sub.2 should be best kept below 325.degree. C. That is, however, all but easy to realize, as the temperature profile inside the lamp depends in a complex way on factors such as working position: horizontal, vertical or any intermediate position, and dimensions and constituting materials of possible lamp housings.
Finally, the oxygen release rate of BaO.sub.2 is high only at temperatures in excess of 500.degree. C., so that the maximum allowed temperature of 360.degree. C. is against the request of a rapid release of oxygen at the beginning of lamp life.